The use of ferroelectric materials as capacitor dielectrics in dynamic random access memories may prove to be the single most important advancement in the design of such memories in more than a decade. Ferroelectric materials are characterized by ultra-high dielectric constant values (reportedly, as high as 10.sup.4, vs. 7 for silicon nitride) and by a propensity to undergo spontaneous magnetic polarization in the presence of a sufficiently strong electric field. The polarized state, useful for non-volatile memory applications, is reversible upon the application of an electric field of opposite polarity. Unfortunately, the polarizability of ferroelectric materials is subject to fatigue, which occurs gradually following repeated cycling between the two magnetic states, and is characterized by a shrinking of the hysterisis curve. As a general rule, at some point after some 10.sup.10 bipolar stressing cycles, a ferroelectric material becomes unusable. However, if ferroelectric materials are used as ordinary cell dielectrics within a capacitance-based memory array, and the array is normally utilized in a dynamic mode (i.e., with unipolar stressing of the dielectric without hysteresis cycling), with bipolar stressing of the dielectric being reserved for long-term storage functions (e.g., for machine-off data storage), the fatigue factor becomes totally insignificant.
Of the known ferroelectric materials, one of the most carefully studied in the role of a DRAM dielectric is a material known as lead zirconate titanate (PbZr.sub.x Ti.sub.1-x O.sub.3). Lead zirconate titanate, commonly known as PZT, has a dielectric constant of approximately 577 (vs. 6-7 for silicon nitride), a breakdown voltage of approximately 2.times.10.sup.6 V/cm (vs. 7-8.times.10.sup.6 for silicon nitride), and a leakage current density of approximately 25 .mu.A/cm.sup.2 (vs. approximately 10 nanoamps/cm.sup.2 for silicon nitride). None of the three values is polarity dependent. As can be seen, the leakage current density of PZT is some 2,500 times higher than that of silicon nitride. Although the leakage current density of PZT films may be nearly halved by adding impurities such as lanthanum and iron which compensate for oxygen and lead vacancies prevalent in the film structure, PZT dielectric layers used in DRAMs must be much thicker than silicon dioxide/silicon nitride/silicon dioxide structures now in use to compensate for the great differences between the two films with regard to leakage current density.
The use of PZT dielectrics in dynamic memory arrays will require many changes in conventional memory manufacturing process flows. For first generation dynamic/non-volatile memories using a PZT dielectric, dielectric thickness will need to be approximately 4,000 .ANG.. This figure is expected to decrease to approximately 2,000 .ANG. for second generation devices. In addition, it will be necessary to construct both top and bottom capacitor electrodes from either metal or a refractory metal silicide to prevent diffusion of lead atoms from the PZT layer into the silicon. Finally, an appropriate cell structure must prevent the high dielectric constant of PZT materials from creating unacceptable levels of cell-to-cell capacitance.